This invention relates to a structure and process for forming an electrical contact between polycrystalline silicon and single crystal silicon and, more particularly, relates to a structure and process for forming a sloped topography contact between a layer of polycrystalline silicon and a single crystal silicon substrate.
In the traditional fabrication of semiconductor devices, external electrical contact with active regions within the semiconductor substrate has been made by laying metal lines over the surface of an insulator on the substrate and making physical contact with the active regions through apertures in the insulation. Recently, polycrystalline silicon doped appropriately with p- or n-type impurities has been used in addition to metal to make such electrical contacts. Typically, a polycrystalline silicon layer would be formed over insulation resting on the semiconductor substrate. The conductive polycrystalline silicon would contact the substrate through apertures in the insulation. These contacts are called diffused or buried contacts. Metal lines would then be placed on further layers of insulation placed on the polycrystalline silicon. The added capability of polycrystalline silicon connectors has resulted in new applications and cost savings due to the elimination of process steps, increased yields and denser device geometries made possible by the multilevel connection patterns. However, the use of polycrystalline silicon to make electrical contacts is often accompanied by relatively deep crevices, on the order of 1.5 -2.5 .mu.deep and 2-10 .mu.wide, which result from etching of the single crystal silicon substrate by the etchant used to define the polycrystalline silicon pattern.
The cratered device topography created by the buried contact crevices introduces design inflexibility as subsequent metal lines will either have to avoid these crevices or be subject to random shorting through the intermediate insulation. The irregular contours of the crevices may also adversely affect execution of any subsequent photoresist step, since the photoresist solution will not evenly coat the contours. Thus, it would be highly desirable to obtain sloped topography contact areas which would accept an even coat of photoresist and over which metal lines could be laid without shorting to the underlying polycrystalline silicon.